1. Field of the Invention
Aspects of the present invention relate to a lithium rechargeable battery. More particularly, aspects of the present invention relate to a lithium rechargeable battery including a separator that provides excellent safety in areas such as short circuit resistance and heat resistance.
2. Description of the Related Art
Recently, as portable electronic instruments have been designed to have a low weight and a compact size, batteries used as drive sources for such instruments have also been required to have a low weight and a high capacity. Active and intensive research and development has been carried out with regard to lithium rechargeable batteries, since lithium rechargeable batteries typically have a drive voltage of 3.6V or higher, which is at least three times higher than the drive voltage of Ni—Cd batteries or Ni-MH batteries, which are currently widely used as power sources for portable electronic instruments. Moreover, lithium rechargeable batteries provide a higher energy density per unit weight than do Ni—CD or Ni-MH batteries.
A lithium rechargeable battery generates electric energy by redox reactions occurring during lithium ion intercalation/deintercalation in a cathode and an anode. A lithium rechargeable battery is obtained by using a material capable of reversible lithium ion intercalation/deintercalation as a cathode active material and an anode active material and by introducing an organic electrolyte or polymer electrolyte between the cathode and the anode.
In general, a lithium rechargeable battery includes: an electrode assembly formed by winding an anode plate, a cathode plate and a separator interposed between both electrode plates into a predetermined shape such as a jelly-roll shape; a can for housing the electrode assembly and an electrolyte; and a cap assembly mounted to the top of the can. The cathode plate of the electrode assembly is electrically connected to the cap assembly via a cathode lead, while the anode plate of the electrode assembly is electrically connected to the can via an anode lead.
The separator in a lithium rechargeable battery has the basic function of separating the cathode and the anode from each other so as to prevent a short circuit. Additionally, it is important for the separator to allow the infiltration of an electrolyte necessary for carrying out electrochemical reactions in the battery and to maintain high ion conductivity. Particularly in the case of a lithium rechargeable battery, the separator is required to have additional functions of preventing substances capable of inhibiting such electrochemical reactions from moving in the battery and/or of ensuring the safety of the battery under abnormal conditions. Generally, the separator includes a microporous polymer membrane based on a polyolefin such as polypropylene or polyethylene, or a multilayer membrane including multiple sheets of such membranes. Such conventional separators have a sheet-like or film-like porous membrane layer, and thus show a disadvantage in that if heat emission occurs by an internal short circuit or an overcharge condition, the pores of the porous membrane may become blocked and the sheet-like separator may shrink. If the sheet-like separator is shrunk by such internal heat emission of the battery, the area covered by the separator may be reduced and the cathode and the anode may come into direct contact with each other, resulting in ignition and explosion of the battery.
Such film-like separators can ensure the safety of a battery upon heat emission caused by a short circuit via a so-called shutdown action that interrupts lithium ion movement (i.e., current flow) by blocking the pores of the separator with a softened polypropylene or polyethylene resin. However, the separators are still disadvantageous when an internal short circuit occurs. For example, using a nail test (penetration) as a substitutive test simulating an internal short circuit condition, it can be shown that the heat emission temperature may locally reach several hundred degrees C. depending on the test conditions, and thus that the porous membrane layer is deformed by the softening or loss of the resin. Further, in the nail test, the test nail penetrates through the cathode and the anode, thereby causing an abnormal overheating phenomenon. Therefore, a separator membrane that utilizes the aforementioned shutdown effect with a softened resin cannot provide an absolute safety measure against an internal short circuit.
Additionally, lithium dendrites may be formed totally on a film-like separator upon overcharge of the lithium rechargeable battery. This is because there is typically a gap between the anode and the film-like separator, and thus, lithium ions that cannot infiltrate into the anode accumulate in the gap between the anode and the film, resulting in the precipitation of lithium metal. If lithium precipitation occurs over the whole surface of the film, such lithium dendrites may penetrate through the film-like separator so that the cathode comes into direct contact with the anode. At the same time, side reactions may occur between lithium metal and the electrolyte to cause heat emission and gas generation, resulting in the ignition and explosion of the battery.